The present invention relates to a data reproduction circuit for reproducing data recorded on a magnetic recording medium, based on an output signal of a floppy disk apparatus which has a pre-erase type magnetic head.
In a floppy disk apparatus, a data reproduction circuit for reproducing data recorded by an MFM (Modified FM) recording system has a data separator using a PLL (Phase Locked Loop) circuit. The data separator separates data pulses and clock pulses from read data output from a floppy disk drive (FDD) and then outputs these pulses. The PLL circuit oscillates in synchronism with the read data and outputs a window pulse that indicates the timing for detecting the data and clock pulses.
The arrangement and operation of a conventional data reproduction circuit will be explained below referring to FIG. 1. In FIG. 1, when a read command is output from a host system (not shown), a floppy disk controller (FDC) (not shown) outputs to a flip-flop 2 a permit signal RG for permitting a read operation. Upon reception of the permit signal, flip-flop 2 becomes operable.
Read data RD, which has been read out from a magnetic recording medium (disk) by a magnetic head of a FDD and converted into a pulse signal, is supplied to a zero pattern detector (SYNC area detector) 1. The zero pattern detector 1 detects clock pulses, recorded as "0" data of a predetermined number of bytes in SYNC areas of a target track, from the read data RD. When detecting the clock pulse train recorded in the SYNC areas, zero pattern detector 1 outputs a detection signal ZS to a clock terminal of a flip-flop 2. Flip-flop 2 in turn outputs a read gate signal RG1 in synchronism with the rising of the detection signal ZS. Upon reception of the read gate signal RG1, a PLL circuit 3 oscillates, locking on the read data RD. PLL circuit 3 outputs window pulses WP to a data separator (not shown) synchronizing with the clock pulses from the SYNC areas and read data. Based on the window pulses from PLL circuit 3, the data separator separates the data pulses and the clock pulses from the read data with the window pulses. The separated data pulses and clock pulses are delivered to the FDC (not shown).
Recently, there have been developed high density recording disks (perpendicular recording type disks) which use a magnetic material such as cobalt-chromium (Co-Cr) and barrium-ferrite (Ba-Fe). The high density recording disks are constituted by a base coated with the magnetic material such as Co-Cr. Reproduction of signals having a high frequency from the high density recording disks requires a magnetic head with a short gap length in order to reduce a gap loss of the magnetic head. However, the shorter the gap length, the smaller the magnetization region in the thickness direction of the disks. With the present state-of-the-art coating technique of the magnetic material, it is difficult to make the coated thickness of the magnetic material to below 1 .mu.m. Therefore, the magnetic head with a short gap length may not be able to perform magnetization saturation recording over the entire region of the high density recording disk in its thickness direction. If the magnetization saturation recording cannot be performed over the entire disk region in the thickness direction, new recorded signals are recorded on the disk with old signals which have been recorded partially remaining thereon. That is, the overwrite characteristic (data rewriting characteristic) for the disk would be deteriorated.
As a solution to this problem, a pre-erase type magnetic head with the structure as shown in FIG. 2 (bottom view) has been proposed. As illustrated in FIG. 2, this magnetic head is constituted by a read/write head 8 and an erase head 9, which is located prior to the head 8 in the rotational direction (arrow Z) of a disk and is combined with the former head through a separator 10. Erase head 9 has a wider erase gap 9a of long gap length while read/write head 8 has a narrow read/write gap 8a of short gap length. In FIG. 2, D is the distance between erase gap 9a and read/write gap 8a. The pre-erase type magnetic head uses erase head 9 to widely and deeply erase data on a track and then uses read/write head 8 to record data on the erased track. According to the pre-erase type magnetic head, therefore, the overwrite characteristic is not deteriorated even if the gap length of the read/write head is short.
For a pre-erase type magnetic head, it is desirable that an erase operation should be completed a predetermined time before completion of an operation for recording data on a magnetic recording medium. Normally, however, erase head 9 and read/write head 8 are simultaneously turned on or off. Consequently, read/write gap 8a and erase gap 9a on a track at the time the magnetic head is ON or OFF would have positional relationships as illustrated in FIGS. 3B and 3C. As shown by arrows A in FIG. 3B, therefore, DC erased areas (areas left erased or erased areas), which have been erased by erase head 9 and in which no data are written by read/write head 8, exist on each track of the magnetic recording medium. These erased areas exist in gap 3 are provided to prevent overlapping of sectors, as shown in FIG. 3A, and corresponds to a period the distance D between the erase gap and read/write gap divided by the linear velocity V of the magnetic recording medium on the track.
In conventional FDCs, an internal command is provided in consideration of a tunnel-erase type magnetic head, by use of which no DC erased areas are provided on the medium. Therefore, the conventional FDCs are not provided with a function to cope with the DC erased area. If noise pulses are reproduced form the DC erased area, this may cause PLL circuit 3 to oscillate locking on the noise pulses. The noise pulse are generated with an irregular frequency. When PLL circuit 3 locks on the noise pulses therefore, the locked frequency deviates significantly from the regular or controllable lock range of the PLL circuit 3, and the circuit 3 overruns. And thus, the FDC detects a read error.
With the use of the conventional FDC, at the time data is reproduced from a magnetic recording medium in which information is recorded using a pre-erase type magnetic head, a PLL circuit in a data reproduction circuit may overrun, thus causing a read error.